Yeah, those are for suicide. Since Martin's dad was only given two, he had to do the deed with the available knife. It's meant to drive home how horrifying the alternative is, though YMMV
And no, Renfri wasn't a witcher. She was just a travelling warrior, her brooch being because she's, not as a mark of any schoolformer royalty
So was Stregobor lying about her being a mutant or something? I remember him saying that his magic wouldn't work on her.